This invention relates to single-substrate optical discs, such as single-substrate audio or video DVDs.
Optical discs having pits and lands that form an optical track structure on one surface are known as single-substrate optical discs, while double-substrate optical discs, in which two single-substrate discs are bonded to each other back to back, are known as dual-substrate optical discs. When a dual-substrate disc is manufactured, the clamp area is characterized by the aggregate thickness of the two bonded substrates. (“Clamp area” is the area of a DVD that is covered or engaged by the clamp mechanism of the reproducing apparatus during play-back of the DVD. The word “area” is frequently used herein to refer to what is actually a three-dimensional portion or region of an optical disc. Thus the clamp area of a disc is not just a two-dimensional surface area, but also the thickness of the disc in that two-dimensional area. A certain volume, quantity, or amount of plastic material is therefore required to make the “clamp area” of a disc.)
In an apparatus for reproducing the information recorded on an audio or video DVD, the clamp that grips the disc must be designed specifically for the thickness of the clamp area of that type of disc. The clamp area of a commercially available 80 mm DVD is usually located within the annular region having an outer radius of about 33 mm and an inner radius (defining the central aperture) of 22 mm.
A typical reproducing apparatus in commercial use today is designed for double-substrate optical audio or video DVDs. The thickness of the clamp area in such double-substrate discs is typically about 1.2 mm, which is composed of the thicknesses of the two bonded substrates, about 0.6 mm each. In order to satisfy the specifications of the clamp designed for use with double-substrate discs, the thickness of the clamp area in a single-substrate optical disc must be made substantially the same as that of a double-substrate disc. For example, if the thickness of the clamp area in a single-substrate DVD is increased to about 1.2 mm, it becomes possible to play a single-substrate 0.6 mm-thick DVD on a player designed for use with bonded 1.2 mm-thick DVDs.
A conventional way of accomplishing this object is by manufacturing an optical disc 10 (FIG. 1) having two regions 20 and 30 of different thicknesses. The first region 20 is a cylindrical region of annular cross section constituting the central portion of the optical disc 10, and the second region 30 is the remaining outer annular region of the disc. The thickness of first region 20 is greater than the thickness of second region 30. Such discs can be created by molding a hub with extra material in the clamp area of the disc, as described in International Application WO 98/10418. If additional material with a thickness of about 0.6 mm is molded onto a substrate 30 with the thickness of about 0.6 mm, the total thickness of region 20 will reach about 1.2 mm. This design makes it possible for a reproducing apparatus designed to play double-substrate DVDs to play single-substrate DVDs as well.
The prior art solution, however, has a variety of disadvantages. For example, when a disc is manufactured according to the design described above, the large amount of material used in forming the hub may cause the hub to shrink radially and vertically, pulling in material from other regions of the disc, including information-carrying regions. This warping phenomenon may negatively affect the tilt, stress, birefringence and electrical properties of the disc. Attempts at reducing this phenomenon, such as increasing cool-down time in the mold for each disc, reduce the efficiency of the disc manufacturing process.